3GPP (3rd Generation Partnership Project) is a project for discussing and preparing specifications of cellular telephone systems based on networks of evolved W-CDMA (Wideband-Code Division Multiple Access) and GSM (Global System for Mobile Communications). In 3GPP, the W-CDMA system has been standardized as the 3rd-generation cellular mobile communication system, and its service is started sequentially. Further, HSDPA (High-Speed Downlink Packet Access) with further increased communication rates has also been standardized, and its service is started. 3GPP is discussing evolution of the 3rd-generation radio access technique (Evolved Universal Terrestrial Radio Access: hereinafter, referred to as “E-UTRA”).
As a downlink communication system in E-UTRA, proposed is an OFDMA (Orthogonal Frequency Division Multiple Access) system for multiplexing users using mutually orthogonal subcarriers. Further, in the OFDMA system are applied techniques such as an adaptive modulation/demodulation-error correcting scheme (AMCS: Adaptive Modulation and Coding Scheme) based on adaptive radio link control (Link Adaptation) such as channel coding, etc. AMCS is a scheme for switching radio transmission parameters (hereinafter, referred to as “AMC mode”) such as an error correcting scheme, coding rate of error correction, the level of data modulation, etc. corresponding to propagation path conditions of each mobile station apparatus so as to efficiently perform high-speed packet data transmission. For example, data modulation is switched to a multilevel modulation scheme with higher modulation efficiency such 16 QAM (Quadrature Amplitude Modulation), 64 QAM, etc. from QPSK (Quadrature Phase Shift Keying) as the propagation path conditions are better, and it is thereby possible to increase maximum throughput in the mobile communication system.
In OFDMA, it is possible to physically divide the communicable region in the frequency domain corresponding to subcarriers and time domain. A combination of some divided regions is referred to as a resource block, one or more resource blocks are allocated to each mobile station apparatus, and communications are performed while multiplexing a plurality of mobile station apparatuses. In order that the base station apparatus and each mobile station apparatus perform communications with optimal quality and rate in response to the request, required is resource block allocation and transmission scheme determination with consideration given to the reception quality in a frequency band corresponding to each subcarrier in the mobile station apparatus and the request of a communication rate in the mobile station apparatus.
The base station apparatus determines the transmission scheme and scheduling, and therefore, to achieve the request, needs to receive feedback of reception quality from each mobile station apparatus. Further, since conditions of the reception quality change every moment with a change in the propagation path, the mobile station apparatus is required to transmit the reception quality information as feedback to the base station apparatus regularly to perform stable communications.
For an uplink communication system in E-UTRA, it has been considered to adopt Single Carrier-Frequency Division Multiple Access (SC-FDMA). The OFDMA system that is multicarrier communication has advantages of being hard to undergo the effect of multipath and capable of using an optimal frequency band with flexibility corresponding to the propagation path, but has the problem that the Peak to Average Ratio is high in transmission signal, and it is difficult to introduce an amplifier with high output in mobile station apparatuses.
In contrast thereto, the SC-FDMA scheme enables a use frequency band to be selected with flexibility while adopting single carrier communication, and is suitable for uplink of E-UTRA. In this case, to achieve single carrier communication, signal transmission resources allocated to each mobile station apparatus are continuous on the frequency axis.
Further, to increase the communication path capacity in E-UTRA, the use of transmission diversity has been proposed such as SDM (Space Division Multiplexing) using MIMO (Multiple Input Multiple Output), SFBC (Space-Frequency Block Coding), or CDD (Cycle Delay Diversity). MIMO is a generic name for the Multiple input/Multiple output system or technique, and has a feature of using a plurality of antennas on the transmission and reception sides, and making the number of branches of input and output of radio signal multiple to transmit. By using MIMO, it is possible to form a plurality of propagation paths as a space by the effect of multipath, and multiplex a plurality of pieces of information to transmit. On the reception side, it is possible to combine power of a plurality of transmission antennas to obtain the reception gain.
In E-UTRA, it is assumed to use MIMO-SDM and transmission diversity on downlink, and the scheme to perform communication is determined in consideration of propagation path conditions between the base station apparatus and mobile station apparatus.
Further, in using MIMO-SDM on downlink, to facilitate separation of a plurality of sequences of information transmitted from antennas, it is considered to beforehand perform precoding on transmission signal sequences. The information of the transmission signal precoding is obtained from propagation path information estimated based on a signal received in the mobile station apparatus, and the base station apparatus cannot obtain the information completely. Therefore, the mobile station apparatus needs to transmit the transmission signal precoding information to the base station apparatus as feedback. Further, since a value of the precoding varies with a change in the propagation path, it is necessary to continuously send back the transmission signal precoding information at appropriate timing in communication of MIMO-SDM.
As described above, on E-UTRA downlink, it is possible to apply a plurality of sequences by applying MIMO to signals transmitted to the mobile station apparatus from the base station apparatus, and it is considered to support one to four sequences as the number of sequences. The number of sequences is determined based on the propagation path between the mobile station apparatus and the base station apparatus, and is calculated in the mobile station apparatus based on a reference signal transmitted from the base station apparatus. Herein, the mobile station apparatus only knows the number of sequences of frequency division transmission signals in Frequency Division Duplex (FDD) adopted in E-UTRA, and to apply MIMO-SDM to a downlink signal in the base station apparatus, the mobile station apparatus is required to transmit the information of the number of sequences to the base station apparatus as feedback. Since the optimal number of sequences between the base station apparatus and mobile station apparatus varies corresponding to conditions of the propagation path, the mobile station apparatus needs to transmit the information indicative of the number of sequences of transmission signals to the base station apparatus in response to a change in the propagation path.
As described above, to actualize SDM communication by MIMO, each mobile station apparatus is required to transmit three kinds of information i.e. the reception quality information, transmission signal precoding information and information of the number of sequences of transmission signals as feedback for the communication path with the base station apparatus. The number of bits, format and transmission frequency required to give each feedback are different from one another, and are varied corresponding to propagation path conditions and conditions of the mobile station apparatus. In other words, it is desired to switch the feedback schemes with flexibility.
As an example, time variations are more moderate in the optimal number of sequences of transmission signals than in the transmission signal precoding information. When scheduling is performed to always concurrently transmit the optimal number of sequences of transmission signals and the transmission signal precoding information, and feedback is performed in accordance with the transmission period of the transmission signal precoding information, the number of sequences of transmission signals that is not varied undergoes feedback many times, and as a result, overhead arises in uplink resources.
Meanwhile, when feedback is performed in accordance with the transmission period of the number of sequences of transmission signals, information of the transmission signal precoding information is insufficient in the base station apparatus. As a result, MIMO-SDM communication is performed by the precoding that is not suitable for the transmission signal, and system throughput decreases. In E-UTRA, it is a significant issue to increase the number of mobile station apparatuses that the base station apparatus can hold, and it is not desired to adopt means for suppressing uplink resources with a limited band or reducing throughput.
As another different example, when the moving speed of the mobile station apparatus changes, the propagation path also changes between the mobile station apparatus and base station apparatus. With the change, rates of changes are also varied in the reception quality information determined by the propagation path, transmission signal precoding information, and the optimal number of sequences of transmission signals. In other words, with the moving speed, periods to transmit these kinds of feedback information are varied, and therefore, it is desired to perform re-scheduling of the reception quality information, transmission signal precoding information, and the optimal number of sequences of transmission signals corresponding to circumstances.
As a method for the mobile station apparatus to transmit to a plurality of kinds of feedback information to the base station apparatus, various techniques have been reviewed with consideration given to each transmission timing. For example, techniques are proposed in Non-patent Document 1 described below to transmit the sequence information of transmission signals alone to the mobile station apparatus as feedback, and then, concurrently transmit the reception quality information and transmission signal precoding information. In this method, since the sequence information of transmission signals is determined earlier, it is possible to reduce the transmission format of the reception quality information and transmission signal precoding information to be transmitted subsequently, and unnecessary bits cannot be transmitted. In other words, it is possible to reduce uplink overhead.
Further, it is proposed in Non-patent Document 2 to concurrently perform coding on all the sequence information of transmission signals, transmission signal precoding information and reception quality information at a beforehand determined bandwidth to transmit. The processing is thereby simplified.
Non-patent Document 1: “Design Aspects of MIMO-Related UE Feedback”, 3GPP TSG RAN WG1 #49, R1-072213, 2007, March
Non-patent Document 2: “Feedback method for CQI, PMI and rank”, 3GPP TSG RAN WG1 #49, R1-073512, 2007, August